Polyethylene terephthalate (PET) has found many packaging applications specifically in the production of soft drink bottles for a long time due in part at least to its high transparency and good mechanical and barrier properties. In recent years consumption of soft drinks, along with other beverages, has increased extensively which has given rise to the production of large quantities of packaging materials such as PET. Besides economic issues with production and disposal, these materials are mainly nonbiodegradable and are subject to some environmental issues. Therefore finding new methods to lower the economic and environmental impacts of such materials is highly desirable. Improving the mechanical properties of PET, without impacting its transparency and barrier properties, makes it possible to reduce the weight of rigid packaging such as bottles.
PET is one of the industrialized aromatic polyester materials, which has been widely applied in the fields of fiber, film and specifically the production of drink bottles due to high transparency and good barrier properties. Since PET is a stable and non-degradable compound in environment, it has been always a big challenge to reduce the economic and environmental impacts of PET-based products. In other words, it is favorable to use less material without losing the properties (light packaging). To do so, the material must have higher mechanical properties in order to retain the shape stability and the properties required by end users. Although there is a large demand for the development of reduced weight packaging systems, relatively low barrier and mechanical properties are the main drawbacks of such packaging.
Nanocomposites are a new class of engineering materials which have found many applications in various industrial fields such as automotive, construction, and packaging due to their excellent properties, low cost and weight. Based on the fact that polymeric materials, such as PET, suffer from lack of thermal stability and low modulus in comparison with other engineering materials such as metals, different types of filler have been incorporated to polymer matrices to overcome these shortcomings. These traditional fillers are usually in the range of micrometers in size.
With the development of nanoparticles and due to their advantages, many researchers have considered using them as nanofillers to reinforce polymer composites. Nanoparticles can provide a large contact area between different phases in the composite which may result in a significant reinforcement effect on polymers. Developing composite materials by addition of fillers to a polymeric matrix can improve many properties of the material. Fillers having a layered structure, such as smectite-type clays and in particular, hectorite, montmorillonite, and synthetic mica have been studied the most as the offer improvements in mechanicals and barrier properties without sacrificing transparency. Numerous studies have focused on the barrier and mechanical performances of nanoclay-PET composites; however, very little improvement has been reported.
The modifiers used to improve the dispersion of smectite-type clays are not thermally stable at processing temperature of PET. The decomposition of these modifiers may lead to degradation of polymer matrix and inversely affect the mechanical and optical properties of the composite. As opposed to smectite-type clays, Kaolin is a layered aluminosilicate in which each layer in the structure comprises two sublayers: an alumina octahedral sheet and a silica tetrahedral sheet that share a common plane of oxygen atoms. Kaolin is extensively used in many industrial applications such as paper, ceramics, paint, rubber, and plastics industries. The asymmetric structure of the kaolin layers create large superposed dipoles and hydrogen bonds between the oxygen atoms on one side of each layer and the hydroxyl groups on the other side of adjacent layer leading to a large cohesive energy between the layers.
As a consequence of this cohesive energy, only some limited organic molecules can intercalate the space between the layers of kaolin. It is well known to those of ordinary skill on the methods and procedures on how to intercalate the space of kaolin layers. For example, imidazolium derivatives can be inserted within the kaolin layers via a melt reaction strategy. Additionally, other recent investigations include (1) the incorporation of organic modified kaolin in poly(vinyl pyrrolidone) via direct intercalation of the poly(vinyl pyrrolidone) into kaolin interlayer spaces at room temperature using a solution; (2) the intercalation of poly(styrene/maleic anhydride) into kaolin via in situ polymerization using kaolin-DMSO as a starting material; (3) creation of a PVC (polyvinylchloride)-kaolin nanocomposite via a solution method that expanded the interlamellar spaces by DMSO prior to intercalation of the PVC chains; (4) the intercalation of polystyrene into kaolin using a kaolin-DMSO intermediate; and (5) the preparation of kaolin-nylon 6 composites by the polymerization of 6-aminohexanoic acid (AHA) in the interlayer space of kaolin) and a commercial nylon 6 in a twin screw extruder). The results indicated that mechanical properties of the various polymer nanocomposites were improved over the polymers themselves.